Anchors for big ships

[vyv_cox]

I had my chain re-galvanised a few weeks ago, so was stretching it out to measure and mark it. Because you have mentioned before about dragging your chain by hand on the yard, I tried mine. OK, mine is 66m of 10mm, and I am delighted to say that there was no way that I could move anything like the full length of chain on the chuckies (small round stones) of our yard.

There's quite a difference between the weights of 8 mm and 10 mm. I wouldn't even have tried with yours.

 

[vyv_cox]

Although given extreme conditions I will be using my CQR .......... or in tandem depending on how you feel it will work....

Best start a new thread on this one! Probably as contentious as the 'no need for an anchor' one.

 

[knuterikt]

It seems highly unlikely that the ship would drag in a southeasterly direction, under the impact of strong southeasterly winds.
Are you sure that you are giving "factual information"?

You spotted an error that have been on this site since the report was published. http://www.aibn.no/Sjofart/Rapporter/2013-08-eng
I must admit I copied it from without spotting the error, I know these waters well we passed this place two days before the grounding.

The text and maps in the report is correct.

I have corrected my post

Some factual information

Just before midnight on 30 July 2009, Full City lost its anchor hold and started dragging under the impact of strong south-easterly winds and high waves.
The vessel was now dragging sideways in a north-easterly direction and rolling strongly in the high seas.
Because it was dragging in a north-easterlydirection quite close to the shore, there was little opportunity to regain control once the vessel had started dragging. The vessel ran aground at Såstein after dragging anchor for 35 minutes

The anchor lost both flukes.



Source: http://www.aibn.no/Sjofart/Rapporter/2013-08-eng

 

[NormanS]

There's quite a difference between the weights of 8 mm and 10 mm. I wouldn't even have tried with yours.

And yet our boats are of similar sizes. Mmmmm.

 

[JumbleDuck]

If chain had no effect rigs would not use it when anchoring, this could save allot of cost and time during rig moves.

I've never seen it claimed that chain has no effect. I just doubt that it's the effect the deadweight boys think it is.

 

[GHA]

If chain had no effect rigs would not use it when anchoring, this could save allot of cost and time during rig moves.

Isn't dyneema used a lot for anchoring rigs now?

 

[RichardS]

Do you have anything intelligent to add to this discussion?

Do you?

Richard

 

[Cruiser2B]

Do you?

Richard

When you initially responded to my first post, I took the effort to clarify my original comments. Did I get a response? Did you counter with an opposing view? No. Nothing. The next thing we see from you, you're insulting me in the third person.

 

[Cruiser2B]

Any links to published papers?

Otherwise it might just be more Internet noise.

http://www.google.com/url?sa=t&rct=...ioGIBQ&usg=AFQjCNH3mFe1NHizCgkfG4uGQsWKuSqxkQ

 

[GHA]

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CCQQFjAA&url=http%3A%2F%2Fwww.dtic.mil%2Fcgi-bin%2FGetTRDoc%3FAD%3DADA093088&ei=r83-Uti8IIbcyQGDioGIBQ&usg=AFQjCNH3mFe1NHizCgkfG4uGQsWKuSqxkQ

What data are you referring to? The test results contain no data re chain friction and with a quick flick through in many cases show the chain off the bottom. There are figures for deck tension but that will include the vertical component of chain weight in the catinery, not just the horizontal component.
You could be more specific in a report of over 200 pages, this report seems to show that large ship anchors will drag, not that the rode friction does all the work.

 

[Cruiser2B]

What data are you referring to? The test results contain no data re chain friction and with a quick flick through in many cases show the chain off the bottom. There are figures for deck tension but that will include the vertical component of chain weight in the catinery, not just the horizontal component.
You could be more specific in a report of over 200 pages, this report seems to show that large ship anchors will drag, not that the rode friction does all the work.

Page 11.

One data summary is described for guidance. Seven parameters are plotted as functions of true anchor drag distance in Figure 33. In the bottom block, horizontal force at the pulling barge, anchor tension, and chain weight on bottom are plotted versus drag distance. The difference between tension measurements is attributed solely to chain drag both on and in the seafloor. As shown by Figure 33, the contribution of the chain to total mooring load is substantial; this was typical for all the tests. At anchor tension equal to zero, the deck force reflects only chain drag. The 20,000 pounds of chain on bottom caused almost 30,000 pounds of drag resistance in the silt at Indian Island. At 58 feet of drag, roughly 10,000 pounds of chain (about 250 feet) in contact with the seafloor causes about 50,000 pounds of drag resistance. The 3,000-pound (nominal) STATO (actual weight 3,500 pounds) provided about 37,000 pounds of resistance at 58 feet of drag. A safe capacity of this anchor in Indian Island silt is 37,000 pounds. With information on the amount of chain in contact with the seafloor, capacities in excess of the basic anchor capacity could be assumed.

The referred data is in figure 33. I admit it's not the best example as the nature of the study was the testing of certain anchors, but I'm unaware of a study into the use of chain alone. As I said before the anchor system comprises both parts. Elsewhere in the paper, the "chain effect" is noted, but not explained. I know this is not a scientific study, but this guy has an interesting way of explaining it: http://www.science20.com/chatter_box/why_anchors_dont_work

 

[GHA]

Page 11.

The referred data is in figure 33. I admit it's not the best example as the nature of the study was the testing of certain anchors, but I'm unaware of a study into the use of chain alone. As I said before the anchor system comprises both parts. Elsewhere in the paper, the "chain effect" is noted, but not explained. I know this is not a scientific study, but this guy has an interesting way of explaining it: http://www.science20.com/chatter_box/why_anchors_dont_work

Well you're changing your tune to something much less contentious there, from "it (the anchor) isn't expected to dig itself in and provide any pull" to "the contribution of the chain to total mooring load is substantial"


And that guy in the link is all over the place.

Conclusion:

An anchoring system works by converting kinetic energy into heat. It can do nothing else.

If that's the case then where does the energy come from to move the boat forward again as the the force decreases on the boat andcatinery deepens.

 

[noelex]

Overall we should be careful concluding much that is relevant from the anchoring technique used by large ships.

Large ships use primitive and small anchors (for their size) and rely much more on the holding power of the chain.

The scale effect is very significant and these procedures do not work well on our much smaller boats.

Our small boat anchoring also seems superior to large ships. I have seen large ships drag on several occasions when I was anchored securely (admittedly the larger ship anchored in deeper water is exposed to stronger wind). I also understand that it is not very unusual for large ships employ engine power to hold station in stronger wind. (can any professional mariner clarify this and how frequently this is needed.)

 

[onesea]

Isn't dyneema used a lot for anchoring rigs now?

As I under stand mainly the deep water stuff where the weight of wire and chain is significant and they need to lighten the weight of the mooring gear. They can also use some truly weird anchors, try looking up suction anchors...

 

[Giblets]

I also understand that it is not very unusual for large ships employ engine power to hold station in stronger wind. (can any professional mariner clarify this and how frequently this is needed.)

Certainly a couple of the large ships anchored off the east of the Isle of Wight in last night's storm reported to VTS that they were using their engines to maintain position. Even so one of the large container vessels dragged and had to lift and re-set.

 

[TimBennet]

Certainly a couple of the large ships anchored off the east of the Isle of Wight in last night's storm reported to VTS that they were using their engines to maintain position. Even so one of the large container vessels dragged and had to lift and re-set.

This is normal; in Hong Kong harbour, if a ship couldn't 'raise steam' and a full crew when the Typhoon warning signal was hoisted, then tugs would be sent by the harbour master to forcibly remove it from one of the harbour moorings and tow the ship off behind some island (usually Lantau in days gone by) and leave them to fend for themselves on their anchors alone. Most of the classic photos you see of ships on the beaches after a typhoon were of these ships.

 

[knuterikt]

Certainly a couple of the large ships anchored off the east of the Isle of Wight in last night's storm reported to VTS that they were using their engines to maintain position. Even so one of the large container vessels dragged and had to lift and re-set.

It's consistent with this information

1.9.2 Design of anchor arrangement (http://www.aibn.no/Sjofart/Rapporter/2013-08-eng)

Design of anchor arrangement
...
The regulations are based on the assumption that the seabed provides a good holding ground for the anchor (A1.1.3), and that, under normal circumstances, a vessel will use only one bow anchor and chain (A1.1.5). The design criteria are based on an assumed current speed of 2.5 m/sec, wind speed of 25 m/sec and that the payed out length of anchor chain is between 6 and 10 times the water depth (A1.1.4).

So it's seems clear that big ship anchor systems is not designed for gale conditions.

 

[Kukri]

This is normal; in Hong Kong harbour, if a ship couldn't 'raise steam' and a full crew when the Typhoon warning signal was hoisted, then tugs would be sent by the harbour master to forcibly remove it from one of the harbour moorings and tow the ship off behind some island (usually Lantau in days gone by) and leave them to fend for themselves on their anchors alone. Most of the classic photos you see of ships on the beaches after a typhoon were of these ships.

Been there, done that thing. (Typhoon "A" buoys)

Ships anchors are not really "the same as they were - in recent decades the Class Societies have approved what are called "HHP" (High Holding Power) anchors, which weigh less than the older Admiralty Stockless type; we also as an industry have pretty much gone over to Kenter shackles in anchor chain.

A ship's anchors and indeed her mooring warps are sized by her Classification Society on the basis of her Equipment Numeral which is calculated on her windage.

The Full City's anchor lost both flukes; this does not happen every day.

 

[knuterikt]

The Full City's anchor lost both flukes; this was a manufacturing defect in the anchor.

Source?

My source say otherwise, from 1.8 Examination of the anchor arrangement (http://www.aibn.no/Sjofart/Rapporter/2013-08-eng)

1.8.1 Material examination
Both flukes were missing from the starboard anchor on board Full City after the vessel had run aground at Såstein. One of the flukes was found on the seabed near the vessel's line of drift. The fracture surfaces of both the anchor and fluke were cut out for mechanical examination at the Norwegian Defence Research Establishment (FFI) at Kjeller, and isotope photography (radiographic examination) at Nammo in Raufoss. None of the examinations carried out at FFI and Nammo detected any anomalies in the strength or quality of the damaged anchor.

 

[noelex]

Certainly a couple of the large ships anchored off the east of the Isle of Wight in last night's storm reported to VTS that they were using their engines to maintain position. Even so one of the large container vessels dragged and had to lift and re-set.

Thanks for the input. I have never been a proffesional mariner so I am reacting to second hand information , but I don't think we need to try and copy large ship anchoring. The very different sizes mean large ship equipment and practices do not scale well to our size vessels.

In addition there are different problems ( for example we have limited crew, but our engines can be started quickly and provide thrust almost immediately)

There is loads of data, tests and experiences that relate to successful anchoring of our (small) yachts and I think this is where we need to concentrate to improve. The different procedures and principals involved in anchoring large ships should not influence our practices significantly.